The phrase is used in written English in two distinct ways. Firstly a writer may employ the phrase to warn a reader that what he has just, or is just about to read, may be false or deficient in some way. The writer is at pains to ensure the reader knows the possible problems in the work. Secondly an author may use the phrase before issuing a general warning to the reader, relating to the content of the piece, rather than the written details themselves.
Examples
See also